1. Field of the Invention
The present invention relates to a detention unit for a winch retrieving an elongated body, e.g., a cable, such as a seismic cable, to avoid damage to the elongated body by local high tension and too hard spooling of the cable at reels or winch drums.
2. Description of the Related Art
In the retrieval of elongated bodies such as cables and the like from the sea onboard a boat, the cables are normally retrieved by a winch and stored on storage reels or are reeled in on a winch drum for storage. Such devices are used for rope and wire of high strength. For more sensitive equipment, such as a seismic cable, however, the high tension and the pressure on the cable on the winch drum or reel while reeling up the cable, may result in a damaged cable. To solve this problem different detention units has been developed.
A known detention unit used e.g. for rope, comprises two discs facing each other to defined a wedge between them. This device gets a good grip on the rope if the tension is great so that the line to be retrieved is wedged between the discs. However, the pressure will lead to deformation and possible destruction of a seismic cable if it were pulled through the device.
Another solution comprises a frustoconical drum on which the elongated body is wound one or more turns. The rope, fishing net or longline to be retrieved enters the drum at the largest diameter and leaves the drum at a lesser diameter, to keep the elongated body in place. This device is not applicable for a sensitive cable, as the cable has to move relative to the drum and may be turned and thus damage the cable.
Another known approach is to use interconnected winches, where the cable to be retrieved is sequentially placed around the winches. To get adequate friction between the cable and the winches, the cable must either be turned several times around each winch or more than two winches have to be used. None of these solutions is acceptable for practical or economical reasons. A solution using several winches is both heavy and expensive, whereas several turns around each winch results in the above-mentioned problem relating to the movement of the cable against the winches.
Another solution used today for cables that can withstand rough handling, is a linear system where the cable is pulled by opposing wheels or conveyor belts. Using this type of device may result in a situation where the cable is starting to run out, because more tension is built up in the starting portion of the system than in the end of the system. This results in undesirable tension and strain on the cable and may result in jerky displacement of the cable. Thus, this solution is not applicable for sensitive cables.
JP 01.176.797, DE 2.631.723, GB 2.294.442 and U.S. Pat. No. 5.152.506 teach pulling in a cable by squeezing it between two endless belts. This solution may result in unacceptable pressure at sensitive parts of a seismic cable in addition to unacceptable local tension on the cable.
SE 364.930 teaches a device for retrieval of a cable, having an endless belt to retrieve the cable and pull the cable towards a reel or winch drum. The device does not provide sufficient detention for the cable and if the tension becomes great the cable might slip against the conveyor and may be damaged.
U.S. Pat. No. 5.082.248 teaches a device having an endless belt pulling, guiding, and pressing the cable towards a drum. This device is not sufficiently gentle for a sensitive seismic cable.
GB 1.566.904 teaches a device using an endless track helically wound around a plurality of drive shafts and where the elongated body is pulled in by means of the endless track. The endless track provides a large contact surface between the track and the elongated body. If the tension becomes too great the elongated body might slip against the track, producing high local tension that may damage the cable.